What is ESD?

ESD stands for electrostatic discharge. Static electricity is a natural phenomenon that can be generated by contact, friction or induction between appliances. Influenced by many factors, electrostatic charge accumulates until it comes into contact with a conductor, at which point the charge is suddenly released, or until it reaches a critical value and even breaks down the surrounding dielectric, quickly restoring charge balance. This rapid release and neutralisation of charge is electrostatic discharge. Static electricity is characterised by long accumulation, high voltage, low charge, small current and short duration.

The harm caused by ESD

Static electricity causes serious damage in many fields. In the semiconductor industry in particular it has become a quality killer for integrated circuit products. Electrostatic discharge accounts for about 40 percent of IC failure cases. Unstable operation and even damage to electronic and electrical products is most often caused by electrostatic damage to their electronic components.

As the IC industry develops rapidly and production processes mature, device integration keeps rising and feature sizes keep shrinking, but the ESD robustness of electronic components has fallen sharply. The extremely thin conductive paths in a device often cannot withstand the high ESD voltages involved. An electrostatic discharge can last from a few nanoseconds to several hundred nanoseconds, reach several hundred or even several thousand volts, and carry several amperes or even tens of amperes. Under such high voltage and current, electronic components suffer irreversible damage, causing obvious product damage or impaired function.

ESD test methods

ESD has many causes. For integrated circuit products, the common ESD events are classified into three types: the human body model (HBM), the machine model (MM) and the charged device model (CDM).

Human body model

This model describes the case where a person accumulates static charge through friction or other means and, when touching an electronic component, discharges that charge through the component pins into the device and then to ground. The model represents a charged human body touching a device, with an equivalent body resistance of 1.5 kΩ and an equivalent body capacitance of 100 pF. The equivalent circuit is shown below.

Figure 1. HBM ESD equivalent circuit model

The HBM current waveform is shown below. Consumer electronic products generally have to withstand 2 kV HBM ESD, with a peak current of about 1.2 A to 1.48 A, a rise time of 2 to 10 ns and a duration of 130 to 170 ns, so the discharge is very fast.

Figure 2. HBM ESD current waveform

HBM ESD sensitivity levels for components are defined in JS-001 and divided into nine classes.

Table 1. HBM ESD sensitivity classes

Machine model

This model simulates electrostatic discharge from a charged conductor onto an electronic component. A machine such as a robotic arm accumulates static charge and, when it touches an IC, discharges through the IC pins. The discharge is shorter and the current higher. Because machines are mostly metal, their impedance can be regarded as 0 Ω, so the equivalent resistance in the machine model is 0 Ω and the equivalent capacitance is 200 pF. Compared with HBM, the equivalent circuit also includes an inductance.

Figure 3. MM ESD equivalent circuit model

The machine model current waveform is shown below. Because there is no resistance in the discharge path, the discharge is shorter, lasting from a few nanoseconds to tens of nanoseconds, and the storage capacitance is larger than in the human body model, so several amperes of discharge current flow. At the same voltage, the machine model produces a larger discharge current than the human body model and is far more destructive to components.

Figure 4. MM ESD current waveform

MM ESD sensitivity levels for components are defined in the JEDEC standard and divided into three classes.

Table 2. MM ESD sensitivity classes

The MM ESD test method is now largely obsolete. The AEC-Q quality certification requirements, for example, have removed the MM ESD test requirement, and only some companies still use the method during product development.

Charged device model

Through friction or other causes, an electronic component accumulates static charge during production and transport, but because the charge is not released quickly the component is not damaged. When the pins of such a charged component approach or touch a conductor or a person, the charge inside the device discharges instantly, and the discharge may last only a few nanoseconds.

Figure 5. Simplified schematic of CDM ESD test equipment

CDM discharge is shorter and the current peak higher, so the device experiences greater ESD stress. Compared with the other two models, CDM is more likely to damage an IC.

Figure 6. CDM ESD current waveform

CDM ESD sensitivity levels for components are defined in JS-002 and divided into five classes.

Table 3. CDM ESD sensitivity classes

ESD requirements in AEC-Q certification

AEC-Q automotive electronic certification sets corresponding ESD test requirements for integrated circuit chips, discrete devices, passive components and other assemblies.

AEC-Q100, AEC-Q101, AEC-Q102, AEC-Q103 and AEC-Q104 all base HBM and CDM ESD testing on JS-001 and JS-002. Unlike the other series, AEC-Q200 uses the ISO 10605 method, with contact discharge and air discharge modes.

Contact discharge applies to semi finished electronic products or products with metal enclosures that can be touched. It simulates the damage that human body discharge during production, transport and use may cause to electronic products.

Air discharge targets products with an insulating enclosure or an insulating protective coating on the surface. The discharge does not touch the sample directly; instead a high voltage electrostatic pulse breaks through the air and reaches the inside of the product, damaging the electronic product or component.

Figure 7. HBM ESD equivalent circuit model for passive components

Figure 8. Contact discharge and air discharge ESD current waveforms

HBM ESD sensitivity levels for passive components follow AEC-Q200-002, as shown in the table below.

Table 4. HBM ESD sensitivity classes for passive components

The ISO 10605 ESD test method is the same as IEC 61000-4-2, differing only in storage capacitance and discharge resistance. For AEC-Q200 passive component testing, the equivalent resistance is 2000 Ω and the equivalent capacitance is 150 pF. By comparison, the 330 Ω and 150 pF or 330 pF specified in IEC 61000-4-2 are more severe.